NO. 7 FLOCKS OF NEOTROPICAL BIRDS — MOYNIHAN 43 



by the sight and sound of quiet and placid birds. Thus, any species 

 that has become conspicuously active and noisy in order to promote 

 intraspecific gregariousness will almost inevitably tend to attract birds 

 of other species particularly strongly. This seems to be what has 

 happened in the case of the plain-colored tanager. 



There are also some indications that generalized gregariousness is 

 usually more strongly stimulated by the sight and sound of a group 

 of birds than by the sight and sound of a single bird of the same 

 species. Thus, birds of a species that has developed a high degree of 

 intraspecific gregariousness may tend to attract individuals of other 

 species comparatively frequently even when they are not particularly 

 conspicuous. 



A few species of the blue and green tanager and honeycreeper alli- 

 ance seem to be attracted to plain-colored tanagers more strongly than 

 would be expected on the basis of generalized gregariousness alone. 

 Such species seem to have developed "special interspecific preferences" 

 for plain-colored tanagers (see below) . This may help to explain why 

 plain-colored tanagers are joined and followed so very much more 

 frequently than are individuals of any other species of the blue and 

 green tanager and honeycreeper alliance, and by individuals of some 

 species so much more frequently than by individuals of other species ; 

 but it is obvious that plain-colored tanagers are also very attractive to 

 many other passerine birds that have not developed any friendly inter- 

 specific social reactions stronger than generalized gregariousness. 



The relationships between plain-colored tanagers and other species 

 in mixed blue and green tanager and honeycreeper flocks are usually 

 essentially one way. Plain-colored tanagers join and follow individuals 

 of other species very much less frequently than they are joined and 

 followed by individuals of other species. 



There is some evidence, in fact, that plain-colored tanagers may 

 actually dislike being joined or followed by individuals of other 

 species. They sometimes attack or perform intention movements of 

 attacking some other birds with which they are associated in mixed 

 flocks, especially such relatively small birds as variable seedeaters and 

 blue dacnises. They do not, however, show much overt aggressiveness 

 toward the species that join and follow them most frequently and 

 persistently. This lack of aggressiveness is probably due to several 

 causes. It has already been mentioned that the hostile behavior of 

 plain-colored tanagers seems to have become modified and/or reduced 

 in several ways, presumably as an adaptation to promote intraspecific 



